1. Field of the Invention
The present invention relates to an orthogonal acceleration time-of-flight mass spectrometer for use in quantitative analysis of trace compounds, qualitative simultaneous analysis of trace compounds, and structural analysis of sample ions.
2. Description of Related Art
[Time-of-Flight Mass Spectrometer (TOFMS)]
A time-of-flight mass spectrometer is an apparatus for finding the mass-to-charge ratios of ions from the times taken for the ions to reach a detector after a given amount of energy is given to the ions such that they accelerate and fly. In TOFMS, ions are accelerated by a constant pulsed voltage Va. At this time, the velocity v of each ion is given as follows from the law of energy conservation:mv2/2=qeVa  (1)v=√{square root over ((2qeV/m))}  (2)where m is the mass of the ion, q is the electric charge of the ion, and e is the elementary electric charge. The ion reaches a detector spaced a given distance of L after a lapse of time T (flight time).T=L/v=L√{square root over ((m/2qeV))}  (3)
It can be seen from Eq. (3) that the flight time T varies depending on the mass m of the ion. TOFMS is an apparatus that isolates masses utilizing this fact. One example of linear TOFMS is shown in FIG. 1. Furthermore, reflectron TOFMS has enjoyed wide acceptance because the apparatus permits improvement of energy convergence and increase in flight distance by placing a reflectron field between an ion source and a detector. One example of reflectron TOFMS is shown in FIG. 2.
[Orthogonal Acceleration TOFMS]
TOFMS must accelerate ions in a pulsed manner by the ion accelerating region in order to analyze variations in mass-to-charge ratio as the elapsed times from a starting point in time. Therefore, TOFMS has very good compatibility with an ionization method in which pulsed ionization is performed, such as by laser irradiation. However, mass spectrometry ionization methods include numerous ionization methods for producing ions continuously such as electron impact (EI) ionization, chemical ionization (CI) ionization, electrospray ionization (ESI), and atmospheric-pressure chemical ionization (APCI). Orthogonal acceleration time-of-flight mass spectrometry has been developed to combine these ionization methods with TOFMS.
FIG. 3 conceptually illustrates TOFMS using an orthogonal acceleration method (i.e., orthogonal acceleration TOFMS). An ion beam created from an ion source that creates ions continuously is conveyed with kinetic energies of tens of kV continuously to an orthogonal acceleration region. In the orthogonal acceleration region, a pulsed voltage of about 10 kV is applied such that the ions are accelerated in a direction orthogonal to the direction in which the ions are conveyed from the ion source. The times taken for the ions to reach the detector after the application of the pulsed voltage are different according to the masses of the ions. Thus, mass separation is performed. See Japanese Patent No. 3,354,427.